The present invention relates to a switching apparatus, and particularly to a switching apparatus in which insertion loss is small in a band of a high frequency such as higher than GHz band, its electric field applying method, and a switching system.
Recently, a telecommunication system, and particularly a mobile telephone or a radio cellular phone are remarkably developing. For example, in a TDMA type communication system, transmission and reception of a RF signal in a GHz high-frequency band is performed through an antenna. In these systems, usually, one antenna is used alternately for output in a transmission step and for input in a reception step. Accordingly, performance required for a switch in such a system is sufficiently small electric resistance in connection and sufficiently high isolation in disconnection. In a case where the electric resistance is not sufficiently small in connection, insertion loss is produced by generation of Joule's heat, so that consumed electric power becomes large. Further, in a case where isolation is low, signals between two circuits of transmission and reception interfere with each other, so that noise is produced.
As a conventional switching apparatus, for example, an electric switching system using a PIN diode is used. However, it has been known that regarding the semiconductor switch, the larger the frequency band of the RF signal is, the larger the electric resistance becomes, thereby causing the loss.
Further, the loss of the PIN diode is about 1 to 1.5 dB in a practical frequency of 2 GHz. Further, though it is desirable that the isolation is 40 dB and more, the isolation in the present PIN diode is about 15 to 25 dB and it is not enough. Therefore, in this industrial field in which it is expected that the frequency will become higher hereafter, a switching apparatus which is low in insertion loss and high in isolation is required.
Therefore, a switching apparatus using a micromachine system has been proposed. In this example, switching is performed not electrically but mechanically. Accordingly, as long as impedance matching with the RF circuit is performed, the loss and the isolation in the high-frequency band are largely improved compared with those in the semiconductor switch (for example, refer to JP-A-2000-348595 (Page 5, FIG. 1)).
However, in an electrostatic type switch as shown in this example, in order to obtain the practical displacement amount necessary for the mechanical switching, a voltage of several tens of volts is required. Usually, in the mobile telephone, a power source of 5V or less is used. Therefore, a booster generator is necessary to obtain a voltage of 25 to 100V, so that problems of size-up and cost-up are caused.
Therefore, a switch using a piezoelectric actuator has been proposed. By using the piezoelectric actuator, the displacement necessary for switching can be obtained at a voltage of several volts that are further smaller than the voltage in the electrostatic type switch. In this example, only one end of a movable portion is mechanically constrained, and the other end is made free. Namely, a form of a cantilever is shown in this example.
FIG. 4 is a perspective view showing a conventional switching apparatus, in which a piezoelectric element 103 previously polarized in a +Z direction and an elastic plate 102 are superimposed, and a main portion of the switch is composed of a unimorph type piezoelectric actuator. An actuator is fixed on a substrate 101 at a fixed portion 105, whereby such an actuator that one end of the actuator is fixed dynamically and the other thereof is made free, that is, an actuator of a cantilever constitution is formed. Through electrode films (not shown) formed on both surfaces of the piezoelectric element film by a power source 106, an electric field in a direction of −z vertical to the film surface is applied, whereby an expansion in an x-direction is produced. This expansion gives a bending moment to the other end because of the existence of the one fixed end, so that the displacement of the actuator is produced as shown in FIG. 4.
However, in such a form, there is the following problem on manufacture of practical finished items: the free end warps by residual stress in film formation, so that flatness of the switch is impaired (for example, refer to JP-A-11-340702 (FIGS. 2 to 5) and JP-A-2000-348594 (FIG. 1)).
Further, even if a piezoelectric actuator having a fixed—fixed beam constitution is used in order to improve the defect of the piezoelectric actuator having the above cantilever constitution, since both ends of a movable portion are fixed, there is usually little displacement of the actuator. Therefore, in order to improve the displacement property of the actuator having a fixed—fixed beam constitution, there is a switching apparatus in which ingenuity is exercised in a fixing method of both ends (for example, refer to Jpn. J. Appl. Phys. 22. Suppl. 2, 154 (1988)).
However, the improvement in the displacement amount achieved by such method is not large, and a contact in the fixing end for generating a bending moment requires a size of some degree, so that real trial manufacture is difficult, and further, the cost is increased. Therefore, this method is unfit for mass production of the actuator.